1. Field of the Invention
This invention relates to thermal printing and more particularly to a thermal print head with improved thermal performance when pulsed using a Pulse Count Modulation scheme and to a method of designing the same.
2. Background of the Invention
Typical apparatus and operation of thermal printing systems is described in U.S. Pat. No. 4,621,271 issued in the name of Brownstein. FIG. 1 is reproduced from such patent and shows a schematic of a typical thermal printer. In brief, such systems are described as comprising a printer apparatus, a carrier web (often called "donor") containing dye available for transference, a receiver (such as paper), and a print head assembly formed of a plurality of individual thermal elements (often called "pixels" or "dots"). Typically, the receiver is fixedly attached to a drum which rotates in a stepped or continuous fashion as regulated by a timing and registration controller within the apparatus. Printing occurs when the imaging controller for the system directs that a particular thermal pixel be heated to the printing temperature. Typically, such heating occurs by flowing electricity through the resistive element which forms the key active component of the thermal pixel. The resistive element heats the thermal pixel. The heated thermal pixel is in contact with the carrier web (donor), and dye is transferred when the temperature at the donor/receiver interface reaches some critical transfer temperature such as the glass transition temperature of the receiver (in dye diffusion thermal transfer). Transference of the dye can be by diffusion, sublimation or other transfer process. After transfer of the dye to the receiver at the desired density, the imaging controller then directs that the thermal pixel be de-energized, and dye transfer ceases once the temperature at the donor/receiver interface returns to temperatures below the glass transition temperature. If the thermal pixel were not de-energized, the donor would in time "burn", i.e., undergo visco-plastic deformation.
As the system is described above, key variables affecting the rate (and therefore the pixel density and apparatus speed) of dye transfer at a particular pixel include the following: 1) time during which thermal pixel is energized; 2) power density to thermal pixel; 3) initial donor, print head, and receiver temperatures; 4) receiver glass transition temperature; 5) donor "burn" (visco-plastic deformation) temperature; and 6) thermal characteristics of the print head. This patent is directed primarily toward optimization of the thermal characteristics of the print head, although the teachings of this patent can also be used to optimize other elements of the system.
FIG. 2 shows a schematic of a typical print head thermal pixel structure. Key elements include the substrate, 1; thermal insulation under-layer, 2; resistor, 3; electrical lead, 4; and protective film, 5. Although the thermal characteristics of each element affects overall thermal performance of the print head, the element most affecting overall thermal performance is the thermal insulation under-layer, 2. In turn, key variables of the thermal insulation under-layer 2 are its thickness, h, and its thermal conductivity, K.
Prior art includes efforts to optimize the thickness range of the insulation under-layer 2 when the print head is pulsed using Pulse Width Modulation (PWM) schemes where the resistor is turned on at most once per print cycle. See, U.S. Pat. No. 4,672,392 issued in the name of Higeta et al. However, Higeta was restricted to PWM energizing schemes and ignores system parameters such as initial temperatures and other key variables listed above which strongly influence overall thermal performance of the print head.
In the prior art, Pulse Count Modulation (PCM) is known to overcome many of the image defects resulting from PWM energizing schemes by permitting better control of resistor 3 temperature. Among the advantages of PCM systems are: better tone scale, higher optical densities, reduced printing artifacts, and enhanced gray scale without distortion of the dye donor. PCM schemes are more complex than PWM schemes, however. Other trends in thermal printing include faster printing times by reducing line printing times from the current 32 milliseconds per line to under 10 milliseconds per line. Such faster speeds require more energy efficient print heads and receivers with lower glass transition temperatures (T.sub.g). The key to making more energy efficient heads is reducing the thermal conductivity of the thermal insulation under-layer 2. The materials currently used do not approach the lowest possible thermal conductivities available in materials. Thus, the trend will be to using materials with lower thermal conductivity.
As the thermal characteristics of the components are changed, the head performance can be tuned to optimize overall integrated system performance. As discussed above, optimization of the thermal insulation under-layer 2 is key to achieving more efficient thermal characteristics in print head performance. In particular, a need has been felt for thermal print heads designed with the optimal thickness, h, of the insulation under-layer 2 for various thermal conductivity, K, and other key parameter values. Also needed is a method of designing such heads. Since print heads can be further optimized for either optimal response time or optimal power consumption, what is further needed are print heads optimized for either shorter response time performance or lower power consumption performance.